Re: Less of

: 'He is less of a fool than I thought he was.'
: and 'It might be less of a problem ethically'

: How to explain 'less of'?

: I've never seen this kind of grammatical rule.

Is your question grammatical--that is, are you wondering whether these are grammatically correct sentences? The answer to this is, probably so, but you might get some argument; they're both entirely idiomatic, however. Or are you unsure of the meaning of the sentences?